Electrolytic treatment of surfaces



P 1962 B. l. ANDERSSON 3,055,812

ELECTROLYTIC TREATMENT OF SURFACES Filed Oct. 10. 1960 2 Sheets-Sheet 1 Fig.1

P 1962 B. I. ANDERSSON 3,055,812

ELECTROLYTIC TREATMENT OF SURFACES Filed Oct. 10, 1960 2 Sheets-Sheet 2 Fig.2

United States Patent 3,055,812 ELECTROLYTIC TREATMENT OF SURFACES Bengt Ivar Andersson, Torshalla, Sweden, assignor to Nyby Bruks Aktiebolag, Nybybruk, Sweden, a company of Sweden Filed Oct. 10, 1960, Ser. No. 61,743 3 Claims. (Cl. 204--140.5)

This invention relates to the electrolytic surface treat ment, such as polishing and pickling, of a metal object. The invention deals in the first place with the treatment of the inside surfaces of tubes with circular cross-sections, especially very fine tubes, but it can also be applied in the internal and external treatment of tubes with other kinds of cross-section, as well as in the surface treatment of other metal objects, such as plates.

It is known that a metal object can be pickled and polished by electrolysis by fiXing the object as an anode in an electrolyte and allowing a body fixed as a cathode to pass by the metal surface (or allowing the metal object to pass by the cathode), at the same time as the electrolyte is made to flow in the space between the anode and the cathode. Here the surface of the cathode is held parallel with that of the anode so that the density of the current is in the main evenly distributed at all points of the cathode body. This method does not give completely satisfactory results since the surface thus treated has not been altogether even or uniform. For instance, in the polishing of stainless steel objects these irregularities have shown themselves in the form of etching damage, which is injurious because it reduces the resistance of the polished surface to corrosion.

According to the invention it has now been found that an even, uniform surface can be obtained if the cathode is placed in relation to the anodes so that the space between them is widened in the direction of movemment of the cathode and the electrolyte made to flow in the direction of movement of the cathode.

This positive result, which is achieved with the cathode used as described in the invention, is probably due to the fact that the injurious effect of the gas bubbles formed during the polishing process is reduced. During electrolytic treatment a large number of gas bubbles are formed and these disturb the passage of the current in the space betweenthe cathode and anode and should therefore be removed from this space as quickly as possible. This takes place partly because of the flow of the electrolyte and partly because the gas bubbles tend to rise. The most favourable conditions for the removal of the gas bubbles are, therefore, obtained if the object to be treated is-placed vertically and the cathode body raised and the electrolyte made to flow upwards in the space between the anode and the cathode. From the underside of the cathode body, where the space is narrowest and the current density accordingly is greatest, there flows a gas free electrolyte which quickly carries upwards the gas bubbles that have formed in that zone. At the top of the cathode body the space is widest and the current density accordingly at its lowest, so the gas bubbles present in that zone do not have any particularly injurious effect on the process. When the cathode body is raised upwards during the treatment, the upper part will thus cause a preliminary treatment of the metal surface while the finishing treatment, which determines the surface quality, is dealt with by the lower part of the cathode body which in fact operates free from the disturbing efiect of gas bubbles.

The absence of gas bubbles in the zone where the current density is greatest also means that a higher current density can be used, thus giving shorter treatment times than if gas bubbles had disturbed the process.

3,055,812 Patented Sept. 25, 1962 The invention will be described more exactly below by reference to the accompanying drawings. FIG. 1 shows an arrangement for the internal polishing of tubes. FIG. 2 shows on a larger scale the pump for circulating the electrolyte in the arrangement according to FIG. 1. FIG. 3 shows an arrangement for polishing or pickling both sides of a plate.

According to FIG. 1 the tube 1 that will be electropolished internally is by means of a lead 2 connected to the positive pole of a direct current source 15. In the tube a cathode body 3 is arranged in the form of a truncated cone supported by an insulated cable 4 which, since it also serves to convey the current, is preferably of copper. The cable 4 is wound on a drum 5 which is driven by a motor 7. By means of a sliding contact 6 and a lead 16, the cable and thereby the cathode body are connected to the negative pole of the direct current source 15. 1

To ensure that the cathode body 3 is held centrally in the tube 1, it is equipped with projections 17 of a nonconducting substance such as hard rubber which rest against the inside wall of the tube.

During the electropolishing the electrolyte is introduced into the lower end of the tube 1 through a conduit 18. The electrolyte is led out of the upper end of the tube through a pipe 10 to a container 11 in which some of the gas bubbles formed during electrolysis are separated from the electrolyte. From the container 11 the electrolyte is conveyed through a conduit 12 to a circulating pump 8 which can, in principle, be of any type, but which is visualized in the illustrated arrangement as consisting of a jet 19 for the injection of a gas, such as air into the electrolyte. The gas draws the electrolyte fluid along with it, partly through the effect of the injector and partly through the so-called mammoth pump effect. The mixture of fluid and gas bubbles rises up through a stand pipe 20 to a larger pipe or container 21, where the gas and fluid are separated and then the electrolyte, because of the force of gravity, runs through a conduit 13 to a heat exchanger 14 where it can be cooled or heated and then on through a conduit 18 to the bottom of the tube 1.

According to FIG. 2 the air intake jet of the pump, called as a whole 19, consists of an inner jet tube 22 connectedto a compressed air conduit 9, and an outer jet tube 23 which is equipped with a number of openings 24.

The cathode body, preferably of copper, is made'so that its upper end is only as thick as is necessary for the connection of the cable 4 carrying the current. The lower end of the cone should not be larger than required so that the space between the cone and the wall of the tube permits the passage of the desired amount of electrolyte. The length of the cathode body, that is to say the height of the cone, should be such that the surface of the cone corresponds to the desired current density, which is usually 0.5-1.0 a./cm. depending on the type of electrolyte, but which may under favourable conditions assume considerably higher values, vide Example 2.

The arrangement according to FIG. 3 consists of an electrolyte tank 26 of such a size that the plate 27, which is to be polished or pickled, can be suspended from a wire 28 and lowered into the tank by means of a lifting device which is not shown and which allows the plate to be lowered slowly and at a constant speed. The plate is coupled as an anode to a current circuit which is not shown. In the tank two long electrodes 29 are placed horizontally and so arranged one on each side of the plate 27 that the space between each electrode and the plate widens upwards. The electrodes 29 are coupled as cathodes to the current circuit which is not shown.

Under the cathode 29, horizontal pipes 30 are arranged having jets which point upwards. These pipes 30 are connected to a pipe 31 which is connected to the pressure side of a pump 32 which sucks electrolyte from the lower part of the tank through a conduit 33.

As has already been mentioned, during the treatment the plate 27 is lowered slowly into the tank, whereupon a preliminary treatment takes place in the wider portion of the space between the plate and the cathodes and a finishing treatment takes place in the narrowest portion of the space. The flow of the electrolyte is etfected by means of the circulation pump which, at the same time, brings about a circulation in the whole tank.

Although FIG. 3 shows an arrangement for treating a single plate, a number of plates can be treated at the same time if a number of cathodes, preferably with trapezium-shaped cross-sections are arranged beside each other.

Example 1 In the arrangement according to FIG. 1, a pipe of stainless steel of type SIS 2333, AISI 304, Werkstolf No. 4301, 4306, containing 18% chromium and 8% nickel was polished electrolytically. The pipe was 6 meters long and had an inner diameter of 26 mm. A copper body was used as cathode in the form of a truncated cone with a diameter of 19 mm. at the base and a diameter of 8 mm. at the smaller end. The length of the cathode was 265 mm. The electrode body was suspended from a plastic insulated copper cable and was raised at a speed corresponding to a polishing time of 6 min./ surface area.

For the polishing a direct current of 6- 10 volts was used and a current intensity corresponding to an average current density of 0.76 a./cm.

The electrolyte was of the phosphoric acid type and was kept at atemperature of 50 C. measured at the point where the electrolyte entered the pipe. The speed of the electrolyte was about 70 litres per min. The entire inner surface of the tube displayed an even polish with a high gloss and was free from any flaws which could have come from the polishing, that is to say, discolouration, porosity or etching damage.

Example 2 For the polishing of metal wire a copper cathode was used having the shape of a cylinder of a length of 100 millimeters and a diameter of 26 millimeters. The cylinder had a central conical boring having a diameter in the lower end of 100 millimeters and a diameter in the upper end of millimeters. The metal wire was coupled as anode and was wound on a drum and was brought to pass in a downward direction through the boring of the cathode. The electrolyte was brought to pass through the boring in, an upward direction. The polished wire was continually rinsed with running water and was then wound upon a drum.

A stainless wire of the type containing 18% chromium and 8% nickel and having a diameter of 1.5 millimeter was electrolytically polished in this device. The wire was brought to pass through the cathode at a velocity of 0.51 meter per minute, dependent on the desired degree of polishing. The electrolyte was of the type containing sulphuric acid, phosphoric acid and chromic acid. The voltage was 11 volts and the current intensity was 68 amperes, corresponding to an average current density of about 14 a./cm. calculated on the wire surface.

In this description no general data have been given regarding the operating conditions such as temperature, current density or the composition of the electrolyte, since these are common technical knowledge.

The arrangements shown in the drawings can, of course, be varied and supplemented in different ways. For example, they may be provided with a device for regenerating the electrolyte in a manner already known.

What is claimed is:

1. A method of electrolytically treating the surface of a metal object, in which the metal object coupled as an anode and a body coupled as a cathode are moved relative to each other and the electrolyte is directed to flow through the space between anode and cathode, characterised in that said metal object is disposed substantially vertically in the electrolyte bath, in that the cathode body is moved in an upward direction relative to said metal object, in that the cathode body is so placed in relation to the metal object that the space between them widens in the direction of movement of the cathode body and that the electrolyte is brought to flow upwardly in the space between said metal object and said cathode body.

2. A method according to claim 1, for polishing the inner surfaces of tubes, characterised by the fact that the cathode tapers upwards from bottom to top.

3. A method according to claim 2 for polishing the inner surfaces of tubes with circular cross-sections, characterised by the fact that the cathode body is shaped like a truncated cone.

References Cited in the file of this patent UNITED STATES PATENTS 2,048,578 Van der Horst July 21, 1936 2,390,282 Toud Dec. 4, 1945 2,412,186 Whitehouse Dec. 3, 1946 2,725,354 Murry Nov. 29, 1955 2,764,540 Farin Sept. 25, 1956 2,868,705 Baier Jan. 13, 1959 2,970,950 Balimann Feb. 7, 1961 FOREIGN PATENTS 429,206 Great Britain May 27, 1935 

1. A METHOD OF ELECTROLYTICALLY TREATING THE SURFACE OF A METAL OBJECT, IN WHICH THE METAL OBJECT COUPLED AS AN ANODE AND A BODY COUPLED AS A CATHODE ARE MOVED RELATIVE TO EACH OTHER AND THE ELECTROLYTE IS DIRECTED TO FLOW THROUGH THE SPACE BETWEEN ANODE AND CATHODE, CHARACTERISED IN THAT SAID METAL OBJECT IS DISPOSED SUBSTANTIALLY VERTICALLY INTHE ELECTROLYTE BATH, IN THAT THE CATHODE BODY IS MOVED IN AN UPWARD DIRECTION RELATIVE TO SAID METAL OBJECT, IN THAT THE CATHODE BODY IS SO PLACED IN RELATION TO THE METAL OBJECT THAT THE SPACE BETWEEN THEM WIDENS IN THE DIRECTION OF MOVEMENT OF THE CATHODE BODY AND THAT THE ELECTROLYTE IS BROUGHT TO FLOW UPWARDLY IN THE SPACE BETWEEN SAID METAL AND SAID CATHODE BODY. 